Polymer light-emitting devices have been divided into two general types: polymer light-emitting diodes (PLEDs) and polymer light-emitting electrochemical cells (PLECs) [1, 2, 3, 4, 5]. The advantages for PLEDs include fast response and relatively long operating lifetime (with proper packaging). However, low work function cathodes and/or thin interfacial layers (e.g. LiF) between the metal and the emitting polymer layer are required. In contrast, PLECs have relatively low turn-on voltages (approximately equal to the band gap of the luminescent semiconducting polymer), and low work function metals are not required.
One of the serious disadvantages of PLECs, however, is the slow response time (time required for the mobile ions to diffuse during junction formation). A solution to this problem is to “freeze” the junction after ion redistribution [6, 7]. A frozen junction system that operates at room-temperature is necessary for practical use. A limiting disadvantage of PLECs has been the relatively short lifetime compared with PLEDs [8, 9]. In order to combine the good characteristics of PLEDs and PLECs, a new type of emitting system was investigated comprising a luminescent polymer doped with a dilute concentration of an ionic liquid. High performance hybrid polymer light-emitting devices are reported here.
The major difference between PLEDs and PLECs is that the latter possess mobile ions inside the polymer; therefore, the selection of the mobile ions is one of the keys to fabricating high performance PLECs. Previously, the mobile ion systems that have been used fall into three categories. The first is polyethylene oxide (PEO) containing Li- salts [2, 3]. Crown ethers (and derivatives) [9, 10] have also been used in combination with metal salts. Finally, polymers with ionic side chains (polyelectrolyte conjugated polymers) have been used [11, 12,13]. For almost all PLECs, the additives comprise at least 5 weight percent (added for the required mobile ions). More important, these systems involve two-component phase separation with the emitting polymer in one phase and the mobile ions (e.g. dissolved in PEO) in a second phase. To create the p-i-n junction of the LEC, ions must move from one phase into the other; e.g. from the PEO into the luminescent polymer. This phase separation appears to degrade the device performance, especially the lifetime [10]. The phase separation can be attributed to the relatively poor compatibility of the ionic materials (hydrophilic) with host light-emitting polymers (hydrophobic). In order to reduce the phase separation, surfactants or bifunctional additives were introduced into the emitting layer and better performance was reported [14]. Single component PLECS polymers have been fabricated using luminescent polymers with ionic side chains (polyelectrolyte conjugated polymers), but the operating lifetimes were poor [11, 12,13].